In the fields of glass, metal, ceramic, plastic, semiconductor, etc. manufacturing, water-repelling, oil-repelling, anti-fogging, anti-contaminating, durability and various other properties are imparted to manufactured substrates by applying a coating film on the substrate surface. By imparting these properties, the value of the manufactured products can be enhanced.
Among well-known methods of providing a coating film on the substrate surface are dipping, spraying, brushing, spin coating, and printing techniques such as the planographic process, relief printing and screen printing. In these techniques, however, the coating solution is merely put physically on the substrate surface, and the adhesion between the substrate surface and coating film is not sufficiently high. A coating film should have a specific thickness. However, it has been difficult using known method to form a coating film having a thickness at the nanometer level which is uniform and free from pin holes.
Treating the surface of a glass or like substrate with a silane-based surface active material is well known in the art and practiced, for example, when producing a glass fiber-reinforced resin with improved adhesion between the glass fiber and resin.
In a first prior art method, silane-based surface active materials containing a monomethoxysilane, dimethyxysilane or trimethoxysilane group, are used for film formation. Prior to film formation, these surface active materials are subjected to hydrolysis to produce highly reactive silanol groups and also form oligomers which comprise siloxane bonds. A coating is formed by dipping a substrate, on which a film is to be formed, in a solution containing the silane-based surface active material. In another method, it is formed by a turn coating process, in which the solution noted above dropped onto the substrate while the substrate is rotated, the number of rotations being appropriately preset according to a desired thickness. In a further method, the solution is sprayed onto the substrate surface for film formation. Any of the above processes permits formation of a coating film of a silane-based surface active material although the uniformity of the film varies. After film formation, the substrate is baked. The temperature and time of baking, are respectively 100.degree. C. and one hour, as standards. In this process, a methanol removal reaction is caused between methoxy groups of the silane-based surface active material and hydroxyl groups exposed at the substrate surface or by each other methoxy groups of the silane-based surface active material. Thus, chemical bonds are produced between the substrate and silane-based surface active material and also by the silane-based surface active material itself, thus obtaining an organic coating film.
In a second prior art example using a silane-based surface active material, or a silane-based surface active material containing a monochlorosilane, dichlorosilane or trichlorosilane group is used. A coating film is formed on the substrate surface by coating such a surface active material in the same manner as in the first prior art example. Where such surface active material are used, chemical bonds between the substrate and silane-based surface active material or by the silane-based surface active material itself, can be obtained. An organic coating film can be formed without any sintering treatment as in the first prior art example.
Surface active materials comprising chlorotitanium groups may further be used in a manner as in the second prior art example using a silane-based surface active material.
With the dipping process, turn coating process and spraying process in the above first prior art example, however, the substrate and silane-based surface active material are bonded to each other by hydrogen bonds, and the film thickness before sintering is not uniform. In addition, the substrate and silane-based surface active material form covalent bonds, and the silane-based surface active material itself forms covalent bonds. It is thus difficult to form a uniform film on the substrate surface.
In the second prior art example film formation process, the silane-based surface active materials comprise chlorosilane groups, which are highly reactive and can react with moisture in the air to form silanol groups and also form an oligomer containing siloxane bonds. When such a solution is coated by a dipping or like process to form the film, a film is obtained like those in the first prior art example. Further, if a hydrogen chloride removal reaction could be caused between hydroxyl groups at the substrate surface and the chlorosilane groups to form a film by the dripping or like process prior to a reaction between moisture in the air and the chclorosilane groups, a condensation reaction between the silane-based surface active molecules themselves form without any covalent bonds being formed with the substrate, thereby forming a polymer comprising siloxane bonds. The polymer makes the film surface rough and also makes the film thickness non-uniform. Further, a film required to be transparent is made opaque with the formation of the polymer. These problems are also raised with the surface active materials containing a chlorotitanium group.
The present invention seeks to solve the above problems inherent in the prior art, and its object is to provide a method of forming a transparent organic coating film, which has a uniform thickness at the nanometer level.